1. Field of the Invention
The present invention relates to an encoder and a signal adjustment method for the encoder. More particularly, the present invention relates to an encoder that enables easy signal adjustment and is suitable for use in a separate type absolute (ABS) linear encoder in which a scale and a detection head are provided separately or rotary encoder, and also relates to a signal adjustment method for that encoder.
2. Description of the Related Art
An encoder for detecting a position from two-phase analog signals formed by a sine wave and a cosine wave generally includes an interpolation (division) circuit in order to detect a change of the position (the moving amount) that is smaller than a period of the signal, as described in Japanese Patent Laid-Open Publication No. Sho 54-19773. The encoder equally divides the signal period at a predetermined pitch so as to obtain resolution smaller than the signal period.
The interpolation circuit calculates a phase angle θ from the following expression, assuming that the sine wave (phase B) signal and the cosine wave (phase A) signal output from the encoder have a central voltage, amplitude, and a phase difference that are predetermined, as shown in FIG. 1.θ=arctan(sin θ/cos θ)  (1)
However, in the case where the central voltage, amplitude, or phase difference of at least one of the sine and cosine waves is not coincident with the predetermined value, the detected phase angle (the change amount of the position equal to or smaller than the signal period) does not have a constant pitch and an error (interpolation error) coincident with the signal period occurs, as shown in FIGS. 2 to 5. FIG. 2 shows an example of the interpolation error in the case where the phase A signal has an offset of 0.2. FIG. 3 shows an example of the interpolation error in the case where the amplitudes of the phase A signal and the phase B signal are 1.0 and 1.2, respectively. That is, FIG. 3 shows a case where there is an amplitude difference between the phase A signal and the phase B signal. FIG. 4 shows an example of the interpolation error in the case where the phase B signal has a phase error of 10°. FIG. 5 shows an example of the interpolation error in the case where the phase A signal has the amplitude of 1.0 and the offset of 0.05 and the phase B signal has the amplitude of 0.95, the offset of −0.05 and the phase error of −5°. That is, FIG. 5 shows the case where the offset, the amplitude difference, and the phase error occur at the same time.
Thus, in case of using a conventional encoder, it is necessary to adjust a signal from the encoder to have a central voltage, amplitude, and a phase difference that are predetermined, while observing the signal (e.g., a Lissajous waveform of the phase A signal and the phase B signal) on an oscilloscope.
The adjustment while observing a display screen of the oscilloscope requires skills. In order to overcome this problem, an encoder is proposed which includes a comparator for determining whether or not a signal having a predetermined level is obtained and outputting a determination result indicating that the determined signal is not an optimum signal inside (or outside) the encoder, as proposed in Japanese Patent No. 3202316. In this case, it is possible to confirm whether or not a predetermined signal is obtained, without using the oscilloscope.
However, the encoder described in Japanese Patent No. 3202316 does not have a self-adjustment function. Therefore, in the case where a range of a signal level in which the comparator determines the signal as the optimum signal is made smaller, mechanical adjustment during attachment of the encoder becomes more difficult. On the other hand, in the case where the above range is made larger, a tolerance of a signal error also becomes larger, resulting in increase in the interpolation error.
On the other hand, an ABS encoder that can detect an absolute position is widely used in machine tool or industrial machinery that includes an encoder for the reasons that (1) the ABS encoder does not require zero return at starting and (2) the ABS encoder does not require a magnetic-pole detector of a linear motor when the encoder is used for feed-back of the linear motor, and other reasons. The ABS encoder includes a multi-track encoder, and synthesizes signals obtained from multiple tracks so as to obtain the absolute position.
Synthesis of the signals from the multiple tracks uses a CPU. An encoder including the CPU therein has been commonly used.